When i had my monitors delivered (see sig) i only had a 270X, my 290X wasnt working (needed mobo bios update) and i was getting 55-60fps around Nurby in the Z4 i think, and my resolution is the exact same number of pixels as 4K monitors.
Im not entirely sure what game gfx settings were used, pretty sure i hadnt touched them but they may have been auto, but it certainly wasnt fulgy thats for sure. If it was dialed back, i cant say i noticed, and i had 2 mates round (1 being 'Mr Fantastic') and the pair of them can be right slapped arses (and they were at the time!) about anything they dont have, and they didnt moan about the visuals. That said, its possible it could have been lower, but not to the detriment of the games great visuals.
Just fired it up, with the 290X. Havent changed any settings, but there may be some sort of auto thing going on.
Screenshot (7.05mb, uncompressed png)
https://dl.dropboxusercontent.com/u/20670693/Obutto/acs_2014_09_05_21_06_07_978.png
Seems i disabled FPS data for screenshots, but the in-game data is there. Dont recall seeing it leave 65fps each time i glanced across at it, i purposely took the screenshot there knowing its a busy part of the track, and yet somehow the game reported 70.5fps.
Im sure it'd take a beating with AI, but i was extremely surprised how well it coped with a resolution comparable (in pxls) to 4K, especially with the 270X. Might not be great online with others, but still quite remarkable how it gets away with it, for such a good looking game.